The invention relates to a resonator unit for an apparatus for heating containers, such as preforms for example, in particular plastic preforms, to an expansion process in which such a resonator unit is used, and to such an apparatus for heating containers.
In the beverage-producing industry, there is an increasing tendency to use, instead of glass bottles, other containers such as for example plastic containers and in particular containers made from PET (PET=polyethylene terephthalate) for beverages. During the production of these containers, firstly preforms are provided, these are heated and are supplied to an expansion process, for example a stretch-blowing process, in order in this way to obtain finished beverage containers. It is customary in the prior art to allow the preforms to run through a heating section, within which they are usually heated by means of infrared radiation.
In addition, however, it is also known in the prior art to use microwave radiation to heat preforms. An apparatus for this purpose is shown schematically in plan view in FIG. 6A and in cross-section in FIG. 6B. The apparatus 1 comprises a microwave generation device or magnetron 4, in which a heating device (not shown) can be integrated. The microwaves are generated in the magnetron 4 and are conducted into a circulator 32. From this circulator 32, the microwaves are introduced by means of a coupling-in device (not shown) into a conducting device 6 in the form of a hollow microwave conductor or hollow rectangular conductor. From there, the microwaves pass via a coupling-in region 12 into a resonator unit 16 and to the preforms 10 arranged within the resonator unit 16.
The temperature of the preforms 10 can be measured by means of a temperature sensor (not shown), such as a pyrometer for example, which is arranged on the resonator unit and in particular measures contactlessly the temperature of the preforms 10. The microwaves coming back from the preforms pass once again into the circulator 32 and from there into a water load 38. The water load 38 serves for damping the microwaves. The returning microwave energy can be measured by means of a sensor device (not shown), such as a diode for example. The measured values can be picked up by a control device (likewise not shown) and used for the power or energy tuning of the microwave power or energy. However, it is also possible to use for the power or energy tuning, in addition to or instead of the values measured by the sensor device, the values output by the temperature sensor for measuring the temperature of the preforms 10. In addition, the values measured by the temperature sensor could also be used to vary the heating phase of the preforms 10.
The power or energy tuning of the microwave power or energy reaching the preforms takes place by means of energy tuning units 14 which in each case consist of a drive device 26, for example in the form of a linear motor, and a regulating body or tuning pin 24. The regulating bodies or tuning pins 24 are arranged on the conducting device 6 in such a way that they can protrude into the conducting device 6 to varying lengths. The length of the regulating bodies or tuning pins 24 protruding into the conducting device 6 can if necessary be varied by the aforementioned control device during ongoing operation of the apparatus, i.e. while heating of the preforms 10 is taking place, in order thus to regulate the microwave energy applied to the preforms. In apparatuses 1 known from the prior art, usually at least three regulating bodies or tuning pins 24 are used to regulate the microwave power or energy.
Usually, the power applied to the preforms 10 is set prior to start-up of the apparatus 1 and then the apparatus 1 is operated with this set power. The energy tuning units 14 are usually impedance tuning units.
As a result, the preforms 10 for heating in the resonator unit 16 are exposed to an alternating electromagnetic field which excites dipoles within the material of the preforms 10, thereby leading to the heating of the preforms 10.
DE 10 2007 022 386 A1 discloses a heating apparatus for plastic preforms. Therein, the region of the plastic preforms that is to be heated is exposed to microwaves in a resonator for at least part of the temporal duration of the heating process.
DE 10 2006 015 475 A1 describes a process and an apparatus for controlling the temperature of preforms. In this process, cylindrical resonator units are used which have relatively high wall current losses in their structure.
FIG. 6A and FIG. 6B likewise show a cylindrical resonator unit 16 in which a preform 10 is introduced essentially into the centre of the resonator unit 16. If the opening for the preform 10 is in the centre of the resonator unit and if the preform 10 is heated by microwaves in the resonator unit 16, the field distribution of the electromagnetic field that forms in the preform 10 is asymmetrical, as shown in FIG. 6C. This also results in an asymmetrical heating of the preform 10. This means that the side of the preform which is assigned to the coupling-in region 12 is heated to a greater extent. Such a heating leads to an asymmetrical or non-uniformly shaped finished beverage container, which is in some cases unfavourable.
In order to solve this problem, it has been proposed to allow the preform to rotate about its axis, in order in this way to achieve a symmetrical heating of the preform. However, it has been found that even such a rotation of the preform cannot always lead to a sufficiently symmetrical heating of the preform.
Moreover, different resonator units have until now had to be used for preforms having different geometries and wall thicknesses. This causes a complicated and cost-intensive provision of a plurality of different resonator units and a likewise complicated and cost-intensive changeover of the entire resonator units depending on the preforms used in each case. Since the production plants customary at present for such containers comprise a large number of resonator units, for example 40 to 50 items, such a provision and changeover of the resonator units is highly disadvantageous.